For the foregoing reasons, I will propose that subdomain data, if any, be kept separate from the Bech32 coding. It may be either kept in a separate string, or somehow affixed with a special delimiter either before or after the Bech32 representation of the onion. Off-the-cuff, which of these looks best to you?
www:onion19qzypww2zw3ykkkglr4tu9
onion19qzypww2zw3ykkkglr4tu9:www
another-level.www:onion19qzypww2zw3ykkkglr4tu9
(My choice of a delimiter here may be wrong, if we want for the browser’s address bar to translate it. I should think more about this.)